Hydroxy-functional epoxy ester resin and hydroxy-reactive crosslinking agent

ABSTRACT

Novel solvent-based thermosetting composition comprising (a) hydroxy functional epoxy ester resin of number average molecular weight (Mn) between about 1,000 and about 5,000, comprising the reaction product of diepoxide with aliphatic diol and, subsequently, with acid component comprising (i) hydroxy functional acid and, optionally, (ii) fatty acid; and (b) polyfunctional, hydroxy-reactive crosslinking agent, for example, aminoplast crosslinking agent or blocked polyisocyanate crosslinking agent comprising isocyanate groups blocked by reaction with an active hydrogen bearing blocking agent. Coating composition may be formulated as primer sprayable with conventional spraying equipment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of Serial No.685,636 filed Dec. 24, 1984, now abandoned. The subject matter of thisapplication is related to that of Ser. No. 565,320, now U.S. Pat. No.4,550,154, Ser. No. 565,321, now U.S. Pat. No. 4,525,569, Ser. No.565,800, now U.S. Pat. No. 4,504,606, Ser. No. 565,801, now U.S. Pat.No. 4,476,259, and Ser. No. 566,066, now U.S. Pat. No. 4,497,938, eachfiled Dec. 27, 1983, and to Ser. No. 685,637 filed Dec. 24, 1984, nowU.S. Pat. No. 4,626,578.

TECHNICAL FIELD

This invention relates to a novel, solventbased, thermosetting epoxyester coating composition. It relates also to such coating compositionformulated, for example, as sprayable coating composition suitable foruse as an automotive vehicle primer to make coatings which are highlyresistant to corrosion, humidity and solvents.

BACKGROUND

Solvent-based coating compositions are known which can be applied to asubstrate, for example, by spraying, and then cured by baking the coatedsubstrate at an elevated temperature suitable to drive off the organicsolvent and to promote crosslinking reaction. The resulting thermosetcoating, if sufficiently humidity and solvent resistant, can provideaesthetic and functional advantages including corrosion protection forthe underlying substrate.

It is an object of the present invention to provide solvent-basedthermosetting coating compositions comprising hydroxy functional epoxyester resins which are crosslinkable during cure, on the surface of asubstrate.

It is another object of the invention to provide a method of making acoating on a substrate, which coating provides advantageous physicalproperties including, for example, humidity and solvent resistance andcorrosion protection for the underlying substrate. Additional aspectsand advantages of the invention will be apparent from the followingdescription thereof.

DISCLOSURE OF THE INVENTION

According to the present invention, a novel organic solvent basedcoating composition, in addition to solvent and any pigments andadditives such as, for example, catalyst, flow control agents and thelike, comprises:

(A) crosslinkable hydroxy functional epoxy ester resin preferably havinga number average molecular weight (Mn) between about 1,000 and about5,000 and comprising the reaction product of diepoxide chain extendedwith aliphatic diol and subsequently chain terminated with acidcomponent comprising hydroxy functional acid; and

(B) polyfunctional, hydroxy-reactive crosslinking agent selectedpreferably from aminoplast crosslinking agent such as, for example,hexamethoxymethyl melamine, and blocked polyisocyanate crosslinkingagent including, but not limited to, blocked trifunctional isocyanuratering containing polyisocyanates and oligoester modified blockedisocyanates, or a compatible mixture of any of them. The crosslinkingagent is included in an amount such that at the cure temperature of thecomposition, the crosslinking agent will provide between about 0.5 andabout 1.6 hydroxy reactive groups per hydroxy group contributed by theepoxyester resin.

Particularly preferred compositions of the invention are those adaptedto be applied by spraying onto a substrate. Such compositions areespecially useful as a primer coat on the bare, unpolished metal surfaceof an automotive vehicle body panel.

According to another aspect of the invention, a method of making acorrosion, solvent and humidity resistant coating on a substratecomprises applying to the substrate the solvent based thermosettingcoating composition of the invention and subsequently subjecting thecoating to an elevated temperature for a time period sufficient tosubstantially cure the coating layer. Typically, the novel coatingcompositions of the invention can be cured by heating to between about240° F. and about 400° F., for a time period sufficient to yield a curedcoating, for example for about 10 to about 60 minutes. According topreferred embodiments of the invention, the coating composition can besufficently cured for good coating properties by heating to about 240°F. for about 20 minutes but, in addition, such preferred compositionwill tolerate curing at up to about 400° F. for as much as about 60minutes without substantial loss of such advantageous coatingproperties.

The coating compositions of the present invention provide cure-responseadvantages without sacrifice of advantageous physical properties in thecured coating. That is, when applied, for example, over a metallicsubstrate, such as when applied as an automotive vehicle primer coatover sheet steel, cured coatings according to the invention have beenfound to provide excellent adhesion to the substrate, excellent humidityresistance, and excellent corrosion resistance.

Other features and advantages of this invention will become moreapparent from the following, detailed description thereof including thepreferred embodiments and the best mode of carrying out this invention.

DETAILED DESCRIPTION OF THE INVENTION

The hydroxy functional epoxy ester resin employed in the composition ofthe present invention preferably has a number average molecular weight(Mn) between about 1,000 and about 5,000. More preferably, especiallyfor use in sprayable coating compositions of higher solids content, theepoxy ester resin has a number average molecular weight (Mn) betweenabout 1000 and about 3000. The hydroxy functional epoxy ester resin isformed by reacting diepoxide with aliphatic diol to yield a diepoxidefunctional chain-extension reaction product having terminal epoxidegroups and subsequently reacting the epoxide groups of the diepoxidereaction product preferably in approximately 1 to 1 equivalent ratiowith acid component of a primary hydroxy functional acid in chainterminating reaction.

Thermosetting compositions of the invention comprise such epoxy esterresin together with polyfunctional hydroxy-reactive crosslinking agentselected, preferably, from aminoplast crosslinking agent and blockedpolyisocyanate crosslinking agent comprising at least two isocyanategroups blocked by reaction with an active hydrogen bearing blockingagent. The polyfunctional crosslinking agent is included in thecomposition in an amount sufficient to provide between about 0.5 andabout 1.6 reactive groups per hydroxy functionality of the epoxy esterresin.

Each of the above major components of the compositions as well asoptional components and other aspects of the invention are describedhereinafter in greater detail.

A. Epoxy Ester Resin

As described above, the hydroxy functional epoxy ester resin is formedby reacting diepoxide with aliphatic diol in chain extension reaction toform a chain extended diepoxide having terminal epoxide groups and thenreacting the chain extended diepoxide with acid component comprisingprimary hydroxy functional acid in chain terminating reaction.

(i) Diepoxide Reactant

The diepoxide reactant suitable for preparing the hydroxy functionalepoxy ester resin can be any of numerous diepoxides including many whichare commercially available and which will be apparent to the skilled ofthe art in view of the present disclosure. While, ultimately, the choiceof diepoxide reactant for preparing the hydroxy functional epoxy esterresin will depend to an extent upon the particular application intendedfor the final coating composition, terminal diepoxides, that isdiepoxides bearing two terminal epoxide groups, are generally mostpreferred. These are generally more reactive and therefore requiremilder reaction conditions under which undesirable side reactions, forexample, epoxy-epoxy reactions and gelation, can be more easily avoided.

Preferably, the diepoxide has a number average molecular weight (Mn)between about 100 and about 1000, and more preferably between about 100and about 600.

Suitable diepoxide reactants include, for example, bisphenol-Aepichlorohydrin epoxy resins such as the Epon (trademark) series, ShellChemical Company, Houston, Tex., and the DER (trademark) series, DowChemical Company, Midland, Michigan. These diglycidyl ether bisphenol-Aresins and higher molecular weight analogs thereof, are preferred inview of their low cost and commercial availability.

Also suitable are cycloaliphatic diepoxy resins such as, for example,the Eponex (trademark) series, Shell Chemical Company, Houston, Tex.;hydantoin epoxy resins such as, for example, Resin XB2793 (trademark),Ciba-Geigy Corporation, Ardsley, N.Y.; and any of a wide variety ofacyclic or cyclic aliphatic diepoxides such as, for example,1,4-butanediol diglycidyl ether and 4-vinylcyclohexene dioxide and thelike. Other suitable diepoxides are commercially available or arereadily prepared by those skilled in the art and will be apparent to theskilled of the art in view of the present disclosure. Also, it will beunderstood from the foregoing that any mixture of compatible diepoxidesmay be used.

In addition to the diepoxide reactant, a portion of the epoxyfunctionality can be provided by any compatible monoepoxy compound orpolyepoxy compound or mixture of such compounds. Suitable polyepoxidesinclude, for example, those of molecular weight about 200 to about 800.The polyepoxide can be any of the well known types such as polyglycidylethers of polyphenols. These can be produced by etherification ofpolyphenol with epihalohydrin in the presence of alkali. It will berecognized by the skilled of the art in view of the present disclosure,that in some instances, particularly where a coating composition of highsolids content is less important, it may be desirable to incorporatepolyepoxide of higher molecular weight. Preferably, any such polyepoxidecontains free hydroxyl groups in addition to epoxide groups.

While polyglycidyl ethers of polyphenol can be employed, it may bedesirable to react a portion of the reactive sites (hydroxyl or in someinstances epoxy) with a modifying material to vary the filmcharacteristics of the resin. The epoxy resin may be modified, forexample, with isocyanate group containing organic materials or otherreactive organic materials.

Other useful polyepoxides are the novolak resins including, for example,the novolak epoxy resins ECN 1235 (trademark) and ECN 1273 (trademark),Ciba-Geigy Corporation.

According to preferred embodiments of the present invention, epoxidecompounds other than diepoxide compounds provide no more than about 15%and most preferably substantially none of the total epoxidefunctionality in the reactants used to form the epoxy-ester resin.

(ii) Aliphatic Diol Reactant

The aliphatic diol reactants suitable for reaction with the diepoxidereactant in chain extension reaction include numerous commerciallyavailable materials, many of which will be readily apparent to theskilled of the art in view of the present disclosure. Preferredaliphatic diols have the general formula (I):

    OH --R --OH                                                (I)

wherein R is a divalent, aliphatic linking moiety, for example, (CH₂)₂wherein n is preferably from 1 to about 8, and the like. Both saturatedand unsaturated diols are suitable. Inorganic moieties, for examplesulfonyl and the like, also are suitable. Diols of this characterprovide good reactivity with diepoxides described above and provide,ultimately, cured coatings of the invention having excellent physicalproperties, most notably excellent corrosion protection. It will beapparent to the skilled of the art in view of the present disclosurethat R preferably is substantially unreactive with the diepoxide and theacid component employed in preparation of the epoxy ester resin.

Preferably the diol has a number average molecular weight (Mn) betweenabout 60 and about 500, more preferably between about 60 and about 200.Particularly preferred diols include those according to formula (I)above, wherein R is selected from the group consisting of a straight orbranched alkylene or alkylidene moiety of one to about 10 carbons,preferably having three to four carbons. Most preferred are terminaldiols, that is, diols bearing two terminal hydroxy functionality, forexample, 1,6-hexanediol, since these generally are more reactive. Othersuitable aliphatic diols include primary/secondary andsecondary/secondary carbon hydroxy substituted diols. Diols bearingtertiary hydroxyl groups are least preferred due to their lowerreactivity. Preferred diols include, for example polycaprolactone diol,alkyl substituted or unsubstituted propanediol, butanediol, pentanediol,hexanediol, and a mixture of any of them. Preferred aliphatic diolsinclude aliphatic diols of about 2 to 20 carbons, for example, ethyleneglycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,2-ethyl-1,3-hexanediol and the like, or a compatible mixture of any ofthem. Other suitable aliphatic diols include, for example, ether diols,especially those of 4 to about 20 carbons, for example, triethyleneglycol and the like.

Other aliphatic diols suitable for preparation of the epoxy ester resinof the present invention will be apparent to the skilled of the art inview of the present disclosure.

(iii) Acid Component Reactant

Numerous suitable hydroxy functional acids will be apparent to theskilled of the art in view of the present disclosure, including manywhich are readily commercially available. These include C₃ -C₂₆ hydroxyfunctional acids, wherein the acid contains one carboxyl group and oneor more hydroxyl groups and no other functional groups which wouldsubstantially interfere with the preparation of the epoxy ester resin,i.e., which would be substantially unreactive with the chain-extensionreactants described above and with the chain-extension reaction product.Preferred hydroxy functional acids are primary hydroxy functional acidscorresponding to the general chemical formula: ##STR1## wherein R₁, R₂,R₃ and R₄ are the same or different and each preferably is loweralkylene such as methylene or ethylene, and Z and Z' are selectedindependently from hydrogen, hydroxyl, and any other noninterferingfunctionality such as nitrile ester group, halogen, amide, etc. Suitablehydroxy acids which may be employed in the chain terminating reactioninclude, but are not limited to, dimethylol propionic acid which is mostpreferred, bis(hydroxy ethyl) propionic acid, bis(hydroxy propyl)propionic acid, and the like and a compatible mixture of any of them.Preferably, the hydroxy acid contains two or more hydroxyl groups, e.g.,at least one of Z and Z' contains a hydroxyl group.

Optionally, the acid component may further comprise fatty acid. Suitablefatty acids include numerous commercially available fatty acids such as,for example, those derived from or contained in either animal orvegetable fat or oil. Preferred are fatty acids from about 8 to about 18carbons, since these are found to provide flexibility to the curedcoating. Also preferred among the fatty acids are the more saturatedfatty acids, since it appears that olefinic unsaturation in the fattyacid can undergo a polymerization-type reaction between such doublebonds during the synthesis of the epoxy ester resin of the invention.Unsaturated fatty acids are suitable for use, however, such as, forexample, oleic acid, linoleic, linolenic and the like and mixtures ofsuch acids, and can be used together with a suitable inhibitor for thepolymerization-type reaction such as hydroquinone or the like, of whichmany are commercially available and will be apparent to the skilled ofthe art in view of the present disclosure. In addition, aromatic fattyacids are commercially available and can be employed. Preferred for useare the substantially saturated fatty acids such as Soya fatty acidwhich is most preferred, and butyric, lauric, palmitic and stearic fattyacids and the like or a compatible mixture of any of them. These arerelatively inexpensive and have been found to provide good reactivitywith the preferred diepoxides described above. For convenience of use,the fatty acids which are semisolid or liquid at room temperature aregenerally preferred over the solid fatty acids.

The hydroxy functional epoxy ester resin used in the composition of thepresent invention can be made according to techniques well known to theskilled of the art. The chain termination reaction occurs subsequentlyto the chain extension of the diepoxide with diol. Thus, diepoxide anddiol are charged into a suitable reactor and heated. The reactants areused in relative proportions to yield a chain extension reaction productbearing two unreacted epoxy end groups and preferably substantially nounreacted diol functionality. Suitable separation techniques are knownto the skilled of the art for removal of unused reactants. It should berecognized that to assure rapid and/or more complete reaction, it may bepreferred to employ a reaction catalyst. the use of catalyst is found toprovide epoxy ester resin which yields coating compositions ofadvantageous physical properties. Suitable catalysts are commerciallyavailable and include any of the well known catalysts for epoxy-diolreactions such as, for example, sodium carbonate, lithium neodecanoateand other organo metallic catalysts and tertiary amines, such as benzyldimethylamine, which is preferred. Other suitable catalysts will beapparent to the skilled of the art in view of the present disclosure.After completion of the chain extension reaction of diepoxide with diol,the epoxy end groups of the diepoxide reaction product are reacted withthe acid component of a primary hydroxy functional acid in chainterminating reaction in approximately 1 to 1 equivalent ratio. Thisratio is preferred, since excess epoxy could result in gelation of thereaction mixture. The resulting product is the hydroxy functional epoxyester resin.

B. Crosslinking Agent

The crosslinking agent employed in the novel solvent based coatingcompositions of the invention comprises polyfunctional, hydroxy-reactivecrosslinking agent selected, preferably, from aminoplast crosslinkingagents and blocked polyisocyanate crosslinking agents.

(a) Blocked Polyisocyanate Crosslinking Agent

Those embodiments of the novel solvent based coating compositions of theinvention employing blocked polyisocyanate crosslinking agents exhibitexceptional shelf stability even when corrosion inhibiting pigments suchas zinc chromate are used in high concentrations.

As used herein "blocked polyisocyanate" means an isocyanate compoundcontaining two or more isocyanato groups, each of which has been reactedwith a blocking agent which will prevent reaction of the isocyanategroup at room temperature with compounds that conventionally react withsuch groups, and which will permit that reaction to occur at higher(cure) temperatures. In general, the blocked polyisocyanate may beprepared by reacting a sufficient quantity of an active hydrogencontaining blocking agent with the polyisocyanate to insure thatsubstantially no free isocyanato groups are present. The blocking agentmay be selected from numerous materials, hereinafter discussed, whichbear an active hydrogen.

The proper proportion of blocked polyisocyanate crosslinking agent tohydroxy functional epoxy ester resin will depend, in part, upon theproperties desired in the coating to be produced and, in part, upon thedesired cure response of the coating composition (which will depend, inturn, upon the baking schedule to be used in curing the coatingcomposition) and, in part, upon the desired storage stability of thecomposition, that is, upon the desired shelf life. Accordingly, theamount of crosslinker that can be used with the epoxy ester resin variesconsiderably. Preferably the blocked polyisocyanate crosslinking agentis included in compositions of the invention in amounts such that upondeblocking of the blocked isocyanato groups at the cure temperature ofthe composition, the crosslinking agent will provide between about 0.5and about 1.6, preferably between about 0.8 and about 1.3, reactiveisocyanato groups per reactive group on the film forming resin of thecoating composition as described above. Blocked polyisocyanates ofnumerous types may be employed in the compositions of the invention.Particularly suitable blocked polyisocyanates, which will be discussedfurther hereinafter, include blocked polymethylene polyphenolisocyanates, isocyanurate ring containing blocked polyisocyanates andcertain oligoester modified blocked polyisocyanates.

In the preparation of the blocked polyisocyanate crosslinking agent, anysuitable organic polyisocyanate may be used. Representative examplesinclude the aliphatic compounds such as trimethylene, tetramethylene,pentamethylene, hexamethylene, 1,2-propylene, 1,2-butylene,2,3-butylene, 1,3-butylene, ethylidene and butylidene diisocyanates; thecycloalkylene compounds such as 1,3-cyclopentane, 1,4-cyclohexane, and1,2-cyclohexane diisocyanates; the aromatic compounds such asm-phenylene, p-phenylene, 4,4'-diphenyl, 1,5-naphthalene, and1,4-naphthalene diisocyanates, the aliphatic-aromatic compounds such as4,4'-diphenylene methane, 2,4- or 2,6-tolylene, or mixtures thereof,4,4'-toluidine, and 1,4-xylylene diisocyanates; substituted aromaticcompounds such as dianisidine diisocyanate, 4,4'-diphenyletherdiisocyanate and chlorodiphenylene diisocyanate; the triisocyanates suchas triphenyl methane-4,4'4"-triisocyanate, 1,3,5-triisocyanate benzeneand 2,4,6-triisocyanate toluene; the tetraisocyanates such as4,4'-diphenyl-dimethyl methane-2,2',5,5'-tetraisocyanate; and thepolymerized polyisocyanates such as tolylene diisocyanate dimers andtrimers, and the like.

In addition, the organic polyisocyanate may be a prepolymer derived froma polyol including polyether polyol or polyester polyol, includingpolyethers which are reacted with excess polyisocyanates to formisocyanate-terminated pre-polymers. The polyols may be simple polyolssuch as glycols, e.g., ethylene glycol and propylene glycol, as well asother polyols such as glycerol; tri-methylolpropane, pentaerythritol,and the like, as well as mono-ethers such as diethylene glycol,tripropylene glycol and the like and poly-ethers, i.e., alkylene oxidecondensates of the above. Among the alkylene oxides that may becondensed with these polyols to form polyethers are ethylene oxide,propylene oxide, butylene oxide, styrene oxide and the like. These aregenerally called hydroxyl-terminated polyethers and can be linear orbranched. Examples of polyethers include polyoxyethylene glycol,polyoxypropylene glycol, polyoxytetramethylene glycol,polyoxyhexamethylene glycol, polyoxynonamethylene glycol,polyoxydecamethylene glycol, polyoxydodecamethylene glycol and mixturesthereof. Other types of polyoxyalkylene glycol ethers can be used.Especially useful polyether polyols are those derived from reactingpolyols such as ethylene glycol, diethylene glycol, triethylene glycol,1,4-butylene glycol, 1,3-butylene glycol, 1,6-hexanediol, and theirmixtures; glycerol, trimethylolethane, trimethylolpropane,1,2,6-hexanetriol, pentaerythritol, dipentaerythritol,tripentaerythritol, polypentaerythritol, sorbitol, methyl glucosides,sucrose and the like with alkylene oxides such as ethylene oxide,propylene oxide, their mixtures, and the like.

A particular class of aromatic polyisocyanates which may be employed inthe novel solvent based coating compositions of the invention arepolymethylene polyphenol isocyanates having the formula: ##STR2##wherein x equals 1 to 3. The compounds, sold under the tradename "PAPI"by the Upjohn Chemical Company of Kalamazoo, Mich., are particularlyuseful in compositions of the invention, resulting in compositionsexhibiting desirable toughness in the final cured coating.

The active hydrogen containing blocking agents which are reacted withthe above described organic diisocyanates may be selected from numerousblocking agents which will be apparent to those skilled in this art.Preferred blocking agents include, for example, those selected from thegroup consisting of (i) aliphatic, cycloaliphatic and aromatic alkylmonoalcohols; (ii) hydroxyl amines; (iii) oximes; (iv) lactams; and (v)triazoles. Any suitable aliphatic, cycloaliphatic or aromatic alkylmonoalcohol may be used as a blocking agent in accordance with thepresent invention. For example, aliphatic alcohols, such as methyl,ethyl, chloroethyl, propyl, butyl, amyl, hexyl, heptyl, octyl, nonyl,3,3,5-trimethylhexyl, decyl, and lauryl alcohols, and the like may beemployed. Suitable cycloaliphatic alcohols include, for example,cyclopentanol, cyclohexanol and the like, while aromatic-alkyl alcoholsinclude phenylcarbinol, methylphenylcarbinol, and the like. Minoramounts of even higher molecular weight relatively non-volatilemonoalcohols may be used, if desired, to serve as plasticizers in thecoatings provided by the invention. Examples of hydroxyl amines whichmay be employed as blocking agents include ethanol amine and propanolamine. Suitable oxime blocking agents include, for example,methylethylketone oxime, acetone oxime and cyclohexanone oxime. Examplesof lactams which may be used as blocking agents are ε-caprolactam,γ-butyrolactam and pyrrolidone, while suitable triazoles includecompounds such as 1,2,4-triazole, 1,2,3-benzotriazole, 1,2,3-tolyltriazole and 4,5-diphenyl-1,2,3-triazole. Particularly preferred activehydrogen containing blocking agents are methylethyl ketoxime and2-ethylhexanol.

(i) Isocyanurate Ring Containing Blocked Isocyanate Compounds

Within the scope of the above general class of blocked polyisocyanatecrosslinking agents, a particular class or type of blockedpolyisocyanate crosslinking agent which may be employed in the novelsolvent based coating compositions of the invention comprisesisocyanurate ring containing blocked isocyanate compounds. In general,these blocked polyisocyanates may be formed by blocking with theaforementioned blocking agents. These compounds may be formed bycyclotrimerization of difunctional isocyanates. Usually, the reactiondoes not stop in this stage and continues through the formation ofpolyfunctional oligomers or a mixture of such oligomers with a portionof the pure trifunctional polyisocyanate. Mixtures of trifunctionalproduct and various polyfunctional oligomers are commercially available.

A particularly desirable blocked polyisocyanate crosslinking agent isthe blocked form of the pure trifunctional isocyanurate represented bythe following formula: ##STR3## wherein each L is selected independentlyfrom the group consisting of aliphatic, cycloaliphatic and aromaticgroups and combinations thereof and B is the residue of an activehydrogen containing blocking agent. More specifically, this compound isdisclosed in U.S. patent application Ser. No. 368,178 filed April 14,1982, the disclosure of which is hereby incorporated by reference.

(ii) Oligoester Modified Blocked Polyisocyanates

Still further particular blocked polyisocyanates useful as crosslinkingagents in the novel solvent based coating compositions of this inventionare oligoester modified blocked polyisocyanates prepared for aparticular class of oligoester diols and triols. A first type of sucholigoester modified blocked polyisocyanate is prepared from organicdiisocyanates wherein one isocyanato group is more reactive than theother, with the more reactive isocyanato first being blocked with ablocking agent and the remaining isocyanato group then being reactedwith hydroxyl functionality of an oligoester diol or triol as referredto above. The second type of oligoester modified blocked polyisocyanatemay be prepared by reacting oligoester diols from the aforementionedclass of oligoester with an excess of organic diisocyanate so as to forman isocyanato terminated prepolymer followed by blocking of the terminalisocyanato groups of the prepolymer with an active hydrogen containingblocking agent. Each of these materials is particularly useful in thecompositions of the invention and produces final cured coatingcompositions exhibiting outstanding flexibility.

Oligoesters of the type employed in the preparation of thesecrosslinking agents are described in U.S. Pat. No. 4,322,508 issuedMar.30, 1982, the disclosure of which is hereby incorporated byreference. The hydroxy functional oligoesters within the useful class ofmaterial (i) have a number average molecular weight (Mn) between about150 and about 3000, preferably between about 230 and about 1000, (ii)bear 2 or 3 hydroxyl groups per molecule, and (iii) are formed by anesterification reaction between a carboxylic acid and an epoxide. Theesterification reaction products are selected from the group consistingof:

(a) the esterification reaction product of polycarboxylic acid, i.e.,carboxylic acid bearing 2 or more carboxyl groups, and monoepoxide;

(b) the esterification reaction product of polyepoxide, i.e., a compoundhaving 2 or more epoxide groups, and monocarboxylic acid, preferablycontaining no ethylenic unsaturation, and bearing no hydroxyfunctionality;

(c) the esterification reaction product of hydroxy functional carboxylicacid and mono- or polyepoxide, preferably monoepoxide;

(d) the esterification reaction product of monocarboxylic acid andhydroxy functional monoor polyepoxide, preferably monoepoxide; and

(e) mixtures of (a) - (d).

As noted above, the first type of oligoester modified blockedpolyisocyanate crosslinking agent is prepared by (i) reacting organicdiisocyanate bearing one isocyanato group which is more reactive thanthe other with a sufficient amount of an active hydrogen containingblocking agent to react substantially with all of the more reactiveisocyanate groups, thus providing a half-blocked diisocyanate and (ii)reacting this half-blocked intermediate with the above discussedoligoester. The organic diisocyanates employed in this synthesis, aswell as the active hydrogen containing blocking agents, are discussedabove in connection with the preparation of the isocyanurate ringcontaining blocked isocyanate cross-linking agents useful incompositions of the invention. The organic polyisocyanate-blocking agentadduct intermediate is formed by reacting a sufficient quantity of theblocking agent with the organic diisocyanate to insure that one of thetwo --NCO groups on the diisocyanate is reacted. The reaction betweenthe organic diisocyanate and the blocking agent is exothermic and thediisocyanate and the blocking agent are preferably admixed attemperatures no higher than about 80° C., preferably below about 50° C.,to minimize the exothermic effect.

The diisocyanate/blocking agent intermediate is next reacted with theoligoester diol or triol described above so as to react substantiallyall free or unblocked isocyanato groups of the intermediate withhydroxyl groups of the oligoester. This reaction is carried outdesirably at a temperature of about 80°-120° C.

As also discussed above, the second type of oligoester modified blockedpolyisocyanate crosslinking agent useful in the novel solvent basedcoating compositions of the invention is prepared by reacting an excessof organic diisocyanate with an oligoester diol from the above describedclass of oligoesters followed by reaction of the terminal isocyanatogroups formed on the resultant prepolymer with an active hydrogencontaining blocking agent as described above so as to react withsubstantially all the isocyanato groups. The diisocyanate startingmaterial is used in excess in amounts sufficient to insure that theintermediate is isocyanate terminated. Therefore, it is preferable thatthe organic diisocyanates and the dihydroxy functional oligoester bereacted in a molar ratio of from greater than 1:1 up to 2:1. Numerousdiisocyanates of the type described hereinbefore may be employed in thepreparation of this intermediate. While it is not necessary that oneisocyanato group be more reactive than the other, the preparation ofthis type of crosslinking agent does not preclude the use of suchmaterial.

(b) Aminoplast Crosslinking Agent

According to alternative embodiments of the invention, the novel solventbased coating compositions employ hydroxy-reactive polyfunctionalaminoplast cross-linking agents. It will be recognized that compatiblecombinations of suitable polyfunctional aminoplast cross-linkingagent(s) and blocked polyisocyanate crosslinking agent(s) also can beused. Included within the aminoplast crosslinking agents suitable foruse in the coating composition are numerous materials which are wellknown to the skilled of the art including, for example, alkylatedmelamine formaldehyde resins with one to about eight carbon atoms in thealkyl moiety. Other suitable aminoplast crosslinking agents will beapparent to the skilled of the art in view of the present disclosure.Many such crosslinking agents are readily commercially availableincluding, for example, the Resimene (trademark) series, MonsantoCompany, St. Louis, Missouri, the most preferred being Resimene 717(trademark), described as a low temperature cure methylatedmelamine-formaldehyde resin.

In addition, suitable polyfunctional aminoplast crosslinking agents canbe prepared employing conventional techniques. Accordingly, for example,a lower alkanol such as methanol, ethanol, butanol, isobutanol,isopropanol, hexanol, 2-ethylhexanol or the like or a mixture of any ofthem is reacted with a melamine formaldehyde. Preferred crosslinkingagents of this type include butylated melamine formaldehyde resin,methylated/butylated formaldehyde resin and polyalkyl hexamethoxymethylmelamine resin which is most preferred in view of its relatively lowercost, ready commercial availability, and its low reactivity with thehydroxy functional epoxy ester resin of the invention at non-elevatedtemperatures. In this regard, preferred polyfunctional aminoplastcrosslinking agent is substantially unreactive with the epoxy esterresin at or below about 60° C. Other suitable aminoplast crosslinkingagents will be apparent to the skilled of the art in view of the presentdisclosure.

The proper proportion of polyfunctional aminoplast crosslinking agent toepoxy ester resin will depend, in part, upon the properties desired inthe coating to be produced and, in part, upon the desired cure responseof the coating composition (which will depend, in turn, upon the bakingschedule intended for curing the coating composition) and, in part, uponthe desired storage stability of the coating composition, that is, uponthe desired shelf life. Accordingly, the amounts of epoxy ester resinthat can be blended with the crosslinker to form coating compositions ofthe invention may be varied considerably. Preferably, the crosslinkingagent is used in amounts of about 5% to about 40% by weight of the totalresin solids, more preferably about 20% to about 30%.

C. General Discussion - Other Aspects of the Invention and OtherComponents

The coating compositions of the invention provide a cured coating havingthe advantageous physical properties described above, over a wide rangeof cure temperatures and a wide range of solids levels. Morespecifically, the coating compositions according to preferredembodiments of the invention cure at temperatures from as low as about120° C. or less within about 15 minutes or less, and yet cure and sufferno significant loss of advantageous physical properties at temperaturesas high as about 200° C. or more for periods up to about 60 minutes ormore. Considered together with the good storage stability of the coatingcomposition, it can be readily recognized that the present inventionprovides a significant advance in the coating composition art.

It will be within the skill of the art to determine the proper volatileorganic content for a given coating composition of the invention and fora given application. Preferred solvents have relatively low volatilityat temperatures appreciably below their boiling points such that solventevaporation is low during storage and during application of the coatingcomposition to the substrate. A suitable solvent system may include, forexample, toluene, methyl ethyl ketone, isobutyl acetate, xylene,cellosolve acetate, acetone and a mixture of any of them. Other solventswhich may be employed include terpenes, aliphatic and aromatic naphthas,and the like. Additional suitable solvents are commercially availableand will be apparent to the skilled of the art in view of the presentdisclosure. Where the composition is to be formulated as a higher solidscontent primer composition employing aminoplast crosslinking agent, itgenerally is preferred to employ a portion of C-1 to C-8 alcohol solventsuch as, for example, butanol, pentanol, hexanol, and the like or amixture of any of them since these inhibit the crosslinking reaction ofthe polyfunctional aminoplast resin with the epoxy ester resin at roomtemperature and thereby improve storage stability. At elevatedtemperature during cure, the alcohol solvent evaporates and, hence,ceases to inhibit the crosslinking reaction. Preferred solvents alsoinclude, for example, methyl amyl ketone and the like, or a mixturethereof with C-1 to C-8 alcohol such as, for example, a 1:2 mixture byweight of butanol and methyl amyl ketone, respectively.

Any solvent allowed to remain in the cured coating should be inert so asto avoid adverse effect upon the cured coating or upon another coatinglayer used in conjunction with it during the curing process orthereafter. Preferrably, the cured coating is substantially free ofsolvent.

Sufficient solvent is used to reduce the viscosity of the coatingcomposition to a level suitable for application to the substrate in thedesired manner. It can be applied by any conventional method, includingbrushing, dipping, flow coating, spraying, etc. Spraying will generallybe preferred, for example, for applying the compositions as anautomotive primer.

Also preferably included in compositions of the invention employingaminoplast crosslinking agent is any of a variety of acid catalyst knownto the skilled of the art to catalyse the aminoplast crosslinkingreaction, for example, p-toluenesulfonic acid, phosphoric acid, phenylacid phosphate, butyl phosphate, butyl maleate, and the like or acompatible mixture of any of them. Such acid catalyst is most useful forcoating compositions intended for low temperature curing schedulesand/or when highly etherified melamine resins are used such ashexa(methoxymethyl)melamine or the like. Such catalysts are used inamounts which depend, in part, upon the intended baking (curing)schedule. Typically, amounts of about 0.2% to about 3.% are used, morepreferably about 0.4% to about 0.6% by weight of total resin solids. Inaddition to catalyst, flow control agent(s), for example, polybutylacrylate; wetting agent(s), for example, silicone; pigments; pigmentdispersants; corrosion inhibitors, for example, chromate pigments,numerous of all of which are known to the skilled of the art, may beemployed in the coating compositions of the invention. In addition,suitable reactive additives can be used, including, for example, lowmolecular weight diol flow control agents and reactive diluents.

Compositions of the invention, and in particular primers of highersolids content, also may include antisettling or anti-sagging agents tocontrol the thixotropic properties of the composition. Exemplary ofavailable materials suitable for this purpose are Dislon (trademark)6900-20X manufactured by Kusumoto Chemicals, Ltd., Tokyo, Japan and soldby King Industries, Norwalk, CT 06852; Bentone (trademark) 38, N. L.Industries, Highstown, N.J.; and Cab-0-Sil (trademark) M-5, CabotCorporation.

Curing the coating composition requires baking for sufficient time atsufficiently elevated temperature to react the crosslinking agent withthe hydroxy functionality of the epoxy ester resin. The time andtemperature required to cure the coating are interrelated and dependupon the particular epoxy ester resin, cross-linking agent, solvent andother materials, if any, and the amount of each comprising the coatingcomposition. The coating compositions according to preferred embodimentsof the invention, as described above, are found to provide best coatingresults when cured at about 300° F. for about 20 minutes. It is a highlysignificant advantage of the invention, however, that these same coatingcompositions can withstand, for example, temperature as high as about200° C. (390° F.) for periods of time as long as about 60 minutes.Accordingly, great flexibility is provided in both designing andimplementing a curing schedule for parts coated with the coatingcompositions of the invention. Thus, in the assembly of automotivevehicles, for example, vehicles unavoidably held in a curing oven forlong periods of time during unplanned assembly line shut-downs arerecovered with cured and unharmed coatings.

Coating compositions according to the present invention, comprising thenovel crosslinkable hydroxy functional epoxy ester resin of theinvention and blocked polyisocyanate crosslinking agent, especially thepreferred materials described above, are found to afford cured coatingswith improved corrosion resistance, thus representing a significantadvance in the art. A most preferred use of the coating composition ofthe invention is as a sprayable primer for use on a bare metal substratesuch as a household or industrial appliance housing or an automotivevehicle body. Primer compositions typically are pigmented and anypigments commonly included in primer compositions for metal substratesand acrylic dispersion topcoats such as, for example, carbon black, ironoxide, lithopone, magnesium, silicate, silica, barium sulfate, Ti0₂,chrome yellow, calcium chromate, strontium chromate, zinc potassiumchromate any the like may be used. The primer can be pigmented accordingto known methods including, for example, by grinding pigments in aportion of the curable resin and adding to the primer composition. Thepigment-to-binder ratio of the primer may be as much as 4:1 by weight,respectively. It is preferred, however, to use a primer having apigment-to-binder ratio of about 1:1 to 2:1 by weight, respectively.

No special expedients are necessary in formulating the primercompositions of this invention. For example, they may be prepared simplyby incorporating the resinous components in a suitable solvent system.Thus, for example, by suitable mixing or agitation, each resinouscomponent may be dissolved in a solvent and the resulting solutionscombined to form finished primer compositions.

The solvent system may be any suitable combination of organic solventsas described above. For a sprayable automotive vehicle primer thesolvent will comprise preferably about 25 to about 35 percent by weightof the total coating compositions although, of course, larger or smalleramounts may be utilized depending upon the solids content desired.

The primer is generally thinned to from about 65 to about 70 percentsolids content for spraying purposes with conventional thinners such asaromatic hydrocarbons, commercial petroleum cuts which are essentiallyaromatic, and the like, and sprayed on to the metal base and cured. Theprimer is cured at elevated temperatures by any convenient means such asbaking ovens or banks of infra-red heat lamps. Curing temperatures arepreferably from about 135° C. to about 165° C., although curingtemperatures from about 100° C. to about 230° C. may be employed, ifdesired.

The invention will be further understood by referring to the followingdetailed examples. It should be understood that the specific examplesare presented by way of illustration and not by way of limitation.Unless otherwise specified, all references to "parts" are intended tomean parts by weight.

EXAMPLE I

In a suitable reactor are charged 1257 parts of Epon 829 (trademark,Shell Chemical Co.; diepoxide), 901 parts of PCP-0200 (trademark, UnionCarbide; polycaprolactone diol) and 9 gms of benzyl dimethyl amine. Themixture is heated to 190°-200° C. and maintained at this temperatureuntil the weight per epoxy of the mixture has advanced to a value of649. At this point 476 parts of Soya fatty acid and 228 parts ofdimethylol propionic acid and 12 parts of lithium neodecanoate arecharged into the reactor. The mixture is heated to 200° C. andmaintained at this temperature until the acid number drops below 8. Thepolymer then is cooled down to 250° C. and thinned with 715 parts ofethylene gycol monoethyl ether acetate. The resulting resin is anhydroxy functional epoxy ester resin suitable for use in a compositionaccording to the invention. The resin with a Gardner-Holt viscosity of Sat 80% solids and an acid number of 6 results.

EXAMPLES II -V

Epoxy ester resins according to the invention are prepared in the mannergenerally of Example I. The components employed are shown in Table I;all amounts are shown in parts by weight.

                  TABLE I                                                         ______________________________________                                                        Example                                                       Composition       II      III     IV    V                                     ______________________________________                                        Epon 829.sup.1    1324            1324                                        DER 333.sup.2             1257                                                Araldite RD-2.sup.3                     859                                   PCP-0200.sup.4            901           901                                   Oligoester.sup.5  469             469                                         Soya Fatty Acid   582     476           476                                   Dimethyl benzyl amine                                                                           10      9       10                                          Dimethylol Proponic Acid                                                                        278     228     556   228                                   Ethylene glycol monoethyl                                                     etheracetate              715                                                 Methyl Amyl Ketone                                                                              663             587   616                                   Lithium Neodecanoate      12      11    12                                    Percent solids    80      80      80    80                                    Viscosity         W       T       Z     O                                     Acid Number       5       6       7     6                                     ______________________________________                                         .sup.1 Trademark, Shell Chemical Co. (diepoxide; specifically, bisphenolA     epichlorohydrin epoxy resin)                                                  .sup.2 Trademark, Dow Chemical Co. (diepoxide)                                .sup.3 Trademark, Ciba Geigy Corp. (diepoxide)                                .sup.4 Trademark, Union Carbide (aliphatic diol)                              .sup.5 A diol prepared according to the method of Example 1 in U.S. Pat.      No. 4,322,508.                                                           

EXAMPLE VI

A millbase, that is, a composition pigment paste, is prepared bygrinding in a ballmill the following mixture:

    ______________________________________                                        Composition        Parts                                                      ______________________________________                                        Barium Sulfate     1626                                                       Red Iron Oxide     60                                                         Titanium dioxide   105                                                        Silica             75                                                         Strontium chromate 99                                                         Polyethylene Wax   48                                                         Xylene             200                                                        Toluene            240                                                        Methyl Ethyl Ketoxime                                                                            57                                                         Resin of Example I 264                                                        ______________________________________                                    

EXAMPLES VII -X

Coating compositions according to the invention are prepared, each beingadapted for use as a high solids, sprayable, pigmented primer forapplication over bare, unpolished steel automotive vehicle body panelsin an automobile vehicle assembly operation. The coating compositioncomponents are shown in Table II, below. Each coating composition wasreduced with methyl amyl ketone to about 18-25 sec. #4 Ford Cup, 27° C.(80° F.) to provide a preferred spraying viscosity. It will be notedthat use of a drier, as in Examples VII, VIII and IX, is optional, tocatalyse reaction of fatty acid double bonds to provide additionalcrosslinking in the cured resin. In Table II, all amounts are expressedin parts by weight.

                  TABLE II                                                        ______________________________________                                                        Example                                                                       VII  VIII     IX     X                                        ______________________________________                                        Epoxy Ester Resin                                                             of Example I      270    270      270  270                                    Phenyl acid phosphate                                                                           --     1        --   --                                     PTSA              --     --       1    1                                      Millbase of Example VI                                                                          800    800      800  800                                    Resimine 717.sup.1                                                                              110    110      --   --                                     Cymel 325.sup.2   --     --       110  --                                     Cymel 303.sup.3   --     --       --   93                                     6% Manganese Naphthanate                                                      (Drier)           4      4        4    --                                     Butanol           35     35       35   --                                     ______________________________________                                         .sup.1 Trademark, Monsanto Co., St. Louis, MO (low temperature, high          solids methylated melamineformaldehyde resin crosslinking agent).             .sup.2 Trademark, American Cyanamid, Wayne, New Jersey, (highly methylate     melamine formaldehyde resin).                                                 .sup.3 Trademark, American Cyanamid, Wayne, New Jersey,                       (hexa(methoxymethyl)melamine).                                           

EXAMPLES XI -XIV

Additional coating compositions according to the invention, each adaptedfor use as a high solids, sprayable pigmented primer for applicationover, for example, bare, unpolished steel automotive vehicle body panelsin an automotive vehicle assembly operation, are prepared as shown inTable III, below. It should be noted that use of a drier, as in thecoating copositions illustrated in Table III, is optional. The epoxyester resin employed in each coating composition is identified byreference to the Example according to which it was prepared. All amountsare expressed in parts by weight.

                  TABLE III                                                       ______________________________________                                                    Example                                                                       XI    XII      XIII     XIV                                                   Exp. II                                                                             Exp. III Exp. IV  Exp. V                                    ______________________________________                                        Epoxy Ester Resin                                                                           270     270      270    270                                     Phenyl acid phosphate                                                                       1       1        1      1                                       Millbase of Exp. VI                                                                         800     800      800    800                                     Resimine 717.sup.1                                                                          110     110      110    110                                     6% Manganese                                                                  Naphthanate (Drier)                                                                         4       4               4                                       Butanol       35      35       35     35                                      ______________________________________                                         .sup.1 Trademark, Monsanto Co., St. Louis, MO (low temperature, high          solids methylate melamineformaldehyde resin crosslinking agent).         

EXAMPLE XV

A blocked isocyanate crosslinker useful in compositions of the inventionis prepared. In a suitable reactor, 417 parts of PAPI 580 (trademark),The Upjohn Company (Kalamazoo, Mich.) are charged under a nitrogenblanket. 261 parts of methyl amyl ketoxime are added dropwise in aperiod of 30 minutes keeping the temperature below 210° F. by externalcooling. After the addition is completed the temperature is maintainedat 210° F. for an additional hour at which time the complete reaction ofthe isocyanate is verified by I.R. The batch is then thinned with 226parts of methyl amyl ketone. The resulting resin has a viscosity of Z₆₊at 75% solids.

EXAMPLES XVI -XIX

Blocked isocyanate crosslinking agents useful in compositions of theinvention are prepared in the manner generally described in Example XVfrom the components shown in Table IV, in which all components are shownin part by weight.

                  TABLE IV                                                        ______________________________________                                                      Example                                                                       XVI   XVII     XVIII    XIX                                     ______________________________________                                        Desmodur L-2291A.sup.1                                                                        360     360                                                   PAPI - 27.sup.2                  399                                          Desmodur IL.sup.3                       525                                   Methyl amyl ketoxime                                                                          174              261    87                                    Benzotriazole           238                                                   Methyl amyl ketone                                                                            133     150      220                                          % Non-Volatiles 80.0    80.1     75.1   57                                    Viscosity       Z.sub.11/2                                                                            Z.sub.7  Z.sub.2                                                                              Z                                     ______________________________________                                         .sup.1 Trademark, Mobay Chemical Corporation (Pittsburgh, Pennsylvania)       (polyisocyanate)                                                              .sup.2 Trademark, The Upjohn Company (Kalamazoo, Michigan) (polyisocyate)     .sup.3 Trademark, Mobay Chemical Corporation (Pittsburgh, Pennsylvania)       (polyisocyanate)                                                         

EXAMPLES XX -XXIV

Coating compositions according to the invention are prepared, each beingadapted for use as a high solids, sprayable, pigmented primer forapplication over bare, unpolished steel motor vehicle body panels in amotor vehicle assembly operation. The coating composition components areshown in Table V below. Each coating composition is reduced with methylamyl ketone to about 18-25 sec. #4 Ford Cup, 27° (80° F.), to provide apreferred spraying viscosity. It will be noted that use of a drier isoptional, to catalyse reaction of fatty acid double bonds to provideadditional cross-linking in the cured resin. In Table V, all amounts areexpressed in parts by weight.

                  TABLE V                                                         ______________________________________                                                     Example                                                          Composition    XX     XXI    XXII  XXIII XXIV                                 ______________________________________                                        Epoxy Ester Resin of                                                          Example I      270    270    270   270   270                                  Millbase of Ex. VI                                                                           800    800    800   800   800                                  Crosslinker of Ex. XV                                                                        125                                                            Crosslinker of Ex. XVI                                                                              115                                                     Crosslinker of Ex. XVII      115                                              Crosslinker of Ex. XVIII           125                                        Crosslinker of Ex. XIX                   162                                  6% Manganese   4                                                              Naphthanate (drier)                                                           dibutyl tin dilaurate                                                                        1      1      1     1                                          N--methyl pyrrolidone                                                                        20     20     20    20    20                                   ______________________________________                                    

What is claimed is:
 1. An organic solvent based thermosettingcomposition comprising:(A) crosslinkable hydroxy functional epoxy esterresin having a number average molecular weight (Mn) between about 1,000and about 5,000 said resin consisting essentially of(I) a reactionproduct formed by reacting a diepoxide with polycaprolactone diol in achain extension reaction to form a reaction product with terminalepoxide groups and no unreacted diol functionality, and (II)subsequently, reacting the terminal epoxide groups of said reactionproduct in a chain termination reaction with acid component comprisinghydroxy functional monocarboxylic acid, wherein the carboxyl group ofthe hydroxy functional monocarboxylic acid is reacted with epoxide groupof the reaction product, said reaction being in approximately 1:1 epoxyto carboxy equivalent ratio to form said hydroxy functional epoxy esterresin; and (B) polyfunctional, hydroxy-reactive crosslinking agentselected from aminoplast crosslinking agent, blocked polyisocyanatecrosslinking agent comprising at least two isocyanate groups blocked byreaction with an active hydrogen bearing blocking agent, and acompatible mixture of any of them, said crosslinking agent beingincluded in an amount such that at the cure temperature of thecomposition said crosslinking agent will provide between about 0.5 andabout 1.6 hydroxy reactive groups per hydroxy group contributed by saidepoxy ester resin.
 2. The solvent based, thermosetting composition inaccordance with claim 1, wherein said diepoxide is selected from thegroup consisting of bisphenol-A epichlorohydrin epoxy resin, hydantoinepoxy resin, cyclic and acyclic aliphatic diepoxides, and a compatiblemixture of any of them.
 3. The solvent based, thermosetting compositionof claim 1, wherein said diol has a molecular weight of about 60-500. 4.The solvent based thermosetting composition of claim 1, wherein saidhydroxy functional monocarboxylic acid is selected from C₃ -C₂₆ acidsbearing a single carboxyl group, at least one primary hydroxyl group,and no additional functionality which would react substantially with thechain extension reaction product of said diepoxide with said diol. 5.The solvent based thermosetting composition of claim 1, wherein saidhydroxy functional monocarboxylic acid is selected from those of thegeneral formula: ##STR4## wherein n is 0 to 20; R₁, R₂, R₃ and R₄ arethe same or different and each is C₁ -C₄ alkylene; and Z and Z' areselected independently from H, OH, nitrile ester, halogen and amide. 6.The solvent based thermosetting composition of claim 5, wherein at leastone of Z and Z' bears an hydroxyl group.
 7. The solvent basedthermosetting composition of claim 1, wherein said hydroxy functionalmonocarboxylic acid is selected from the group consisting ofdimethylolpropionic acid, bis(hydroxy ethyl)propionic acid, bis(hydroxypropyl)propionic acid, and a compatible mixture of any of them.
 8. Thesolvent based thermosetting composition of claim 1, wherein said acidcomponent further comprises fatty acid, said hydroxy functionalmonocarboxylic acid and said fatty acid being in molar ratio of from 1:0to about 1:1.
 9. The solvent based thermosetting composition of claim 8,wherein said fatty acid is selected from the group consisting of Soyafatty acid, butyric, lauric, palmitic and stearic fatty acid and acompatible mixture of any of them.
 10. A solvent based, thermosettingcomposition in accordance with claim 1, wherein said crosslinking agentconsists essentially of blocked polyisocyanate.
 11. A solvent based,thermosetting composition in accordance with claim 10, wherein saidblocked polyisocyanate is selected from blocked aliphatic, aromatic,cycloalkylene, aliphatic aromatic and nuclear substituted aromaticpolyisocyanates.
 12. A solvent based, thermosetting composition inaccordance with claim 10, wherein said blocked polyisocyanatecrosslinking agent comprises blocked polymethylene polyphenol isocyanatewhich prior to blocking has the formula: ##STR5## wherein x equals 1 to3.
 13. A solvent based, thermosetting composition in accordance withclaim 1, wherein blocked polyisocyanate crosslinking agent is employedwhich comprises the reaction product of an active hydrogen-bearingblocking agent with polyisocyanate comprising isocyanaturatering-bearing polyisocyanate prepared by cyclotrimerization ofdiisocyanate.
 14. A solvent based, thermosetting composition inaccordance with claim 13, wherein said blocked polyisocyanatecrosslinking agent consists essentially of blocked trifunctionalisocyanurate represented by the formula: ##STR6## wherein each L isselected independently from the group consisting of aliphatic,cycloaliphatic and aromatic divalent radicals and B is the residue ofsaid active hydrogen bearing blocking agent.
 15. A solvent based,thermosetting composition in accordance with claim 10, whereinpolyisocyanate employed in the preparation of said blockedpolyisocyanate crosslinking agent comprises an isocyanate terminatedprepolymer prepared by reacting polyol with an excess of polyisocyanate.16. A solvent based, thermosetting composition in accordance with claim1, wherein said crosslinking agent consists of blocked polyisocyanateprepared by(A) reacting (i) organic diisocyanate represented by theformula

    OCN - L' - NCO

wherein L' is selected from the group consisting of aliphatic,cycloaliphatic and aromatic radicals and combinations thereof andwherein one of the isocyanate groups thereof is substantially morereactive that the other, and (ii) sufficient active hydrogen bearingblocking agent to react with substantially all of said more reactiveisocyanate groups; and (B) reacting the reaction product of (A) withsufficient polyol to react with the remaining isocyanate groups.
 17. Asolvent based, thermosetting composition in accordance with claim 16,wherein said polyol is selected from oligoester diols and triols which(i) have a number average molecular weight (Mn) between about 150 andabout 3,000 and (ii) are the product of esterification reaction betweencarboxylic acid reactant and epoxide reactant, said esterificationreaction product being selected from the group consisting of theesterification reaction product of:(a) polycarboxylic acid andmonoepoxide; (b) polyepoxide and monocarboxylic acid containing noethylenical unsaturation and bearing no hydroxy functionality; (c)hydroxy functional carboxylic acid and monoepoxide; (d) monocarboxylicacid and hydroxy functional monoepoxide; and (e) mixtures of (a) -(d).18. The solvent based, thermosetting composition of claim 1, whereinsaid crosslinking agent consists of hexa(methoxymethyl)melamine.
 19. Theorganic solvent based, thermosetting composition of claim 1comprising:(A) crosslinkable hydroxy functional epoxy ester resin havinga number average molecular weight (Mn) between about 1,000 and about5,000, wherein the diepoxide reactant is selected from the groupconsisting of bisphenol-A epichlorohydrin epoxy resin, hydantoin epoxyresin, cyclic and acyclic aliphatic diepoxide and compatible mixture ofany of them, and the primary hydroxy functional organic monocarboxylicacid is selected from the group consisting of dimethylolpropionic acid,bis(hydroxyethyl)propionic acid, bis(hydroxy propyl)propionic acid and acompatible mixture of any of them; and (B) blocked polyisocyanatecrosslinking agent comprising at least two isocyanate groups which havebeen blocked by reaction with an active hydrogen bearing blocking agent,said blocked polyisocyanate being selected from blocked aliphatic,aromatic cycloalkylene, aliphatic aromatic, and nuclear substitutedaromatic polyisocyanates and being included in said composition in anamount such that upon de-blocking of the blocked isocyanate groupsthereof at the cure temperature of the composition, said crosslinkingagent will provide between about 0.5 and about 1.6 reactive isocyanategroups per reactive hydroxy group of said epoxy ester resin.